SPERMATOGENESIS IN THE ALBINO RAT 135 



and various modifications of them, the following fluid gave the 

 most satisfactory results: 



Picric acid, sat. aq. sol. 75 cc. 



Formol (c. p.) 25 cc. 



Glacial acetic acid 5 cc. 



To this mixture (Bouin's fluid), freshly made and raised to a 

 temperature of 38°C., are added and thoroughly dissolved the 

 following, in the order stated: 1.5 grams chromic acid crystals, 

 and then 2 grams of urea crystals. Each of these latter should 

 be dissolved as added, or a precipitate will be formed when the 

 urea is put in. Even then, unless the formol is of a high degree of 

 purity, this white precipitate is likely to occur. In small quantity 

 it does no harm. In passing, it is well to note that all the chem- 

 icals should be the very best obtainable. 



Technique. From an animal that has been killed either by 

 chloroform or by decapitation, the testis is removed as quickly 

 as possible and snipped into small pieces, which are dropped into 

 the fixative as they are cut. The fixative is at the temperature 

 of 38° or 40°C. and is held at that temperature during the first 

 hour of fixation. Fixation is complete in from one to two hours, 

 depending upon the age of the animal. Longer fixation, up to 

 forty-eight hours, although accompanied by more general shrink- 

 age, does not seem detrimental to the chromosomes. 



The fixative is replaced by the drop method with 70 per cent 

 alcohol, the picric acid washed out by the addition to the alcohol 

 of a saturated solution of lithium carbonate added a few drops at 

 a time; the alcohol is replaced by anilin oil (freshly distilled), 

 this by synthetic oil of wintergreen, and this by paraffin of 52° 

 melting point. The oils are changed by the drop method. The 

 paraffin is added very slowly until the tissue is in a bath of high 

 paraffin concentration. It is then passed through several changes 

 of paraffin to remove all of the oil, a process which takes from one 

 to three hours, depending upon the size of the pieces of tissue. 

 All .details of the method, which is very simple, are recorded in 

 my paper upon the study of the effect of fixatives (Allen, '15). 

 Sections were cut at thicknesses of from 2 to 10 micra. Those 

 found most valuable for study were from 5 to 7 micra. The very 



